The application generally relates to positive-displacement compressors. The application relates more specifically to controlling balance piston pressure in a screw compressor.
In a screw compressor, the gas can be drawn, compressed, and discharged by the rotation of a male rotor and a corresponding female rotor. In many screw compressors, the male rotor can be used to drive the female rotor. The predominant force on the male rotor can be from thrust. A portion of the thrust force comes from the pressure of discharge gas acting on the end plane of the male rotor. However, a sizable portion of the thrust force comes from torque transmission between the rotors. If the thrust force is not balanced or otherwise reduced, the male rotor, the female rotor, bearings and/or other components can rapidly wear through friction.
To counteract the thrust force, many screw compressors may use a thrust bearing in conjunction with a balance piston at the opposite end of the rotor. The balance piston can be used to reduce the size and cost of the thrust bearing required to handle the thrust force at full load operation of the compressor.
The balance piston can be a round disk that is tightly fitted to the male rotor and keyed to the rotor. The outer diameter of the balance piston can be grooved to create a labyrinth seal to permit flow but to reduce viscous losses. The outer diameter of the balance piston and the mating surface in the rotor housing can be controlled to extremely tight tolerances to control fluid flow. By applying fluid pressure behind the balance piston, a counteracting or balancing force in the opposite direction to the thrust force can be generated. The size of the balancing force can be dependent upon the diameter of the balance piston and the pressure of the fluid apply to the balance piston.
In many screw compressors, the balance piston and thrust bearing can offset 75% or more of the thrust forces at full load operation of the compressor. However, when the compressor is unloaded, such as by using a slide valve, the rotor load and thrust force can decrease, while the balance piston force can stay relatively constant. If the balance piston force is not reduced to match the reduction in thrust force, the balance piston force can very easily overpower the thrust bearing and cause the thrust bearing to fail. Therefore, many screw compressors may use a pressure control system to regulate the balance piston pressure. The pressure control system can include control algorithms, a regulator, a solenoid valve, a pressure transducer, and a gauge or feedback mechanism to determine the position of the slide valve. A drawback to the pressure control system is that the equipment is expensive, difficult to set up, and can malfunction.
Therefore, what is needed is a system to automatically regulate the balance piston pressure without complicated control schemes and extensive parts lists.